Measuring instrument with diamond coated contacts

ABSTRACT

Precision measuring instruments having highly parallel and wear-resistant contact members and a method for fabricating these contact areas are disclosed. The measuring instruments preferably include two contact members each having diamond coated ceramic substrates. The method for diamond coating the contact members of the measuring instruments includes, for each contact member, preparing a ceramic substrate to high tolerance so that it has two surfaces which are parallel within 0.25 microns, diamond coating one surface of the substrate by any one of several chemical vapor deposition (CVD) techniques, and metalizing the other surface of the substrate for affixing it to the contact member of the measuring instrument. The metalized surface of the substrate is preferably affixed to the contact members of the measuring instrument by brazing, gluing or welding. Preferred embodiments of the measuring instruments with diamond coated contact areas include calipers and micrometers. The provided measuring instruments can be used to measure extremely abrasive materials such as sandpaper and grinding wheels without suffering any appreciable wear to their contact surfaces over a long period of time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to measuring instruments. Moreparticularly, this invention relates to highly accurate measuringinstruments having diamond coated contacts and to a method of applyingthe diamond coating to the contacts.

2. State of the Art

As the science of metrology becomes more precise and exacting, the needfor dependable and consistent measuring instruments has becomeessential. Measuring instruments such as micrometers and calipers havehighly parallel contact surfaces which may be subject to considerablewear, particularly if they are used to measure highly abrasivematerials, such as sandpaper, grinding wheels, etc. Such wear may causeirregularities on the contact surfaces which decalibrate the instrumentand yield incorrect and inconsistent measurements. Thus, where theprecision of a measurement is crucial, it may be necessary torecondition the contact surfaces to make them smooth and parallel and torecalibrate the measuring instrument. Such a procedure, however, inaddition to being costly, removes the instrument from use for the periodof time necessary to machine it and to recalibrate it. Although theirregularities in the contacts of a measuring instrument may be toosmall to be noticed by a user, who may then fail to recalibrate theinstrument, these irregularities may still significantly affect theprecision of its measurements, and cause finished parts to be measuredinaccurately.

In order to minimize the wear on the above discussed measuringinstrument contacts, it is known in the art to provide a layer of hardmaterial, such as tungsten carbide, at the contact surfaces. Tungstencarbide, however, is still subject to frequent wear and decalibrationwhen the instrument is used to measure abrasive materials which areharder than tungsten carbide and/or chemically active with its cobaltbinder. It is therefore desirable to use a harder, chemically inert, andmore wear resistant material as a contact surface. The hardest knownmaterial is diamond. The hardness of diamond combined with its lowcoefficient of friction would make it ideal for use as a contact surfacein measuring instruments. However, there is no apparent way to apply adiamond surface to the contacts of a measuring instrument. While itmight be possible to braze diamond pads to the contact surfaces of amicrometer, this would present several significant problems. First, thecontact surfaces of a micrometer must be perpendicular to the shaft, andparallel to each other, within ten one-millionths of an inch at everypoint. This would require the diamond pads to be almost perfectlyparallel relative to each other and perpendicular relative to the shaft.The hardness of diamond makes it difficult to cut and grind to thisrequired accuracy. Also, the cost of a diamond pad is prohibitivebecause the pad must be fabricated from a larger diamond and machined atgreat expense to this required tolerance of parallel sides.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide measuringinstruments having extremely hard and wear resistant contacts.

It is also an object of the invention to provide measuring instrumentshaving hard and wear resistant contacts which are easily fabricated bythe tool manufacturer.

It is another object of the invention to provide measuring instrumentshaving diamond coated contacts.

It is also an object of the invention to provide a method of coating thecontact surfaces of a measuring instrument with diamond.

In accordance with the objects of the invention, which will be discussedin detail below, a ceramic substrate having two opposed surfaces isprepared to a high tolerance. The ceramic substrate, which may be, forexample, silicon nitride, tungsten carbide, or another formable strongceramic, is shaped so that it has a substantially constant thickness towithin 0.25 microns. One surface of the substrate is then coated with adiamond layer approximately ten microns thick by any one of severalchemical vapor deposition (CVD) techniques. The other surface ismetalized and affixed to one of contacts of a measuring instrument bygluing, brazing, or welding. A second substrate is then prepared in asimilar fashion to the first, and affixed to the other contact of themeasuring instrument.

According to the invention, a measuring instrument is provided whichbroadly includes at least one contact having a ceramic substrate. Oneside of the ceramic substrate is partially coated by any one of severalchemical vapor deposition (CVD) techniques with a thin diamond layer ofabout ten microns thickness. The other side of the substrate ismetalized and affixed to a contact area of the measuring instrument bybrazing, gluing or welding.

Several measuring instruments utilizing the diamond coated ceramicsubstrate of the invention are provided. A first such instrument is adigital micrometer which generally includes a substantially C-shapedframe, an anvil, a spindle, a spindle adjuster, and an electronicmeasuring device. The C-shaped frame has two arms which aresubstantially parallel and form a jaw therebetween. The cylindricalanvil has a flat, generally circular, contact portion. The anvil isaffixed to the end of one of the arms of the frame such that the surfaceof the contact portion of the anvil is perpendicular to the arm andfaces into the jaw. The spindle is axially adjustable and is alignedwith the anvil. The adjustable spindle has a flat contact portion whichis similar in size and shape to the contact portion of the anvil and isarranged to be parallel to the contact portion of the anvil. The spindlehas an axially rotatable handle for adjusting the spindle, and anelectronic measuring device is coupled to the spindle in a manner knownin the art so that it can measure the distance between the contactsurfaces of the anvil and the spindle. In accord with the preferredembodiment of the invention, each of the spindle and anvil contactportions further includes a disk formed from silicon nitride or anothersuitable substrate, with the disk having two flat surfaces. Prior toattaching the disk to the contact portions of the spindle and anvil, thesurface of each disk is metalized to promote adhesion to the spindle andanvil contact portions such as by gluing or brazing; and the othersurface of the disk is CVD diamond coated and forms the outer contactsurface of the spindle and anvil once the disk is affixed to the spindleand anvil.

Another measuring instrument utilizing the invention is a digitalcaliper which generally includes a track having a perpendicular fixedfirst outside measuring jaw with a contact edge portion, an adjustablesecond outside measuring jaw having a contact edge portion and parallelto the first outside jaw, a fixed inside measuring jaw and an adjustableinside measuring jaw, and an electronic measuring device adjustablydisposed along the track. The adjustable outside measuring jaw andadjustable inside measuring jaw are adjusted by rotating an adjustingwheel on the electronic measuring device. Each of the first outsidemeasuring jaw and the second outside measuring jaw contact edge portionsfurther includes a strip formed from silicon nitride or another suitablesubstrate, with the strip having two flat surfaces with a width equal tothat of the outside measuring jaws. Prior to attaching the strips to thejaws, one surface of each strip is metalized to promote adhesion to thefirst and second outside measuring jaws such as by gluing or brazing;and the other surface of the strip is CVD diamond coated so as form thecontact edges of both measuring jaws. In an alternative embodiment ofthe calipers, the ceramic substrate strip may have the same length asthe measuring jaws, with only a diamond coated end portion of the stripforming the actual contact edge.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a detailed view of a diamond coated silicon nitride diskprepared according to the invention for attachment to the anvil andspindle of a micrometer;

FIG. 2 is a perspective view of a micrometer having a diamond coatedanvil contact surface and the diamond coated spindle contact surfaceaccording to the invention;

FIG. 3 is a detailed view of a diamond coated silicon nitride stripprepared according to the invention for attachment to the contact edgesof a measuring jaw of a calipers; and

FIG. 4 is a perspective view of a calipers having diamond coated outsidemeasuring jaw contact edges according to the invention.

FIG. 5 is a perspective view of the contact edge portion of a secondembodiment of a calipers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to FIG. 1, a detailed view of a diamond coated anvil orspindle contact pad 100 for a micrometer (further illustrated anddescribed with reference to FIG. 2) is shown. The contact pad 100preferably includes a silicon nitride disk 102 having an edge 104 andhaving a first surface 106 and an opposite second surface 108. The disk102 is machined or otherwise prepared so that the first surface 106 andthe second surface 108 are parallel to each other within 0.25 microns(approximately ten one-millionths of an inch). The disk 102 preferablyhas a diameter substantially equal to that of the anvil and the spindleof the micrometer to which it is to be applied. A diamond layer 110having a preferred thickness of about ten microns is coated on the firstsurface 106 of the disk 102 and preferably over the edge 104 of the disk102 by any known CVD technique such as disclosed for example in Weimer,W. A. et al., "Examination of the Chemistry Involved in Microwave PlasmaAssisted Chemical Vapor Deposition of Diamond", J. Mater. Res., Vol. 6,No. 10 (Oct. 1991); Bachmann, P. K. et al., "Towards a General Conceptof Diamond Chemical Vapour Deposition", Diamond and Related Materials,Elsevier Science Publishers (1991, pp. 1-12); and Japanese Laid-OpenPatent Application No. 4-272179. The second surface 108 of the disk 102is metalized and affixed to the anvil or spindle of the micrometer bygluing, brazing, welding, or any other suitable technique. In accordwith the invention, the disk 102 may be made of ceramic compounds otherthan silicon nitride, such as silicon carbide or tungsten carbide,provided the ceramic compound can be properly coated with a diamondlayer and is sufficiently strong to act as a contact. Preferably, thedisk 102 is not so hard as to be difficult to machine or prepare to therequired parallel tolerance, and is also not so expensive thatpreparation of the disk is cost prohibitive.

Referring now to FIG. 2, a first exemplary embodiment of a measuringinstrument 200 according to the invention is shown. The measuringinstrument 200, known as a digital micrometer, includes a substantiallyC-shaped frame 202 with a first arm 204 to which a cylindrical anvil 206is affixed, and a second arm 208 which contains the spindle mechanism210, an electronics package (not shown) coupled to the spindle mechanism210 in a manner known in the art, a digital LCD readout 212 coupled tothe electronics package, and an electronic keypad 214 coupled to theelectronics package. The anvil 206 is substantially perpendicular to thefirst arm 204 and has an axis 216 facing into the jaw 218 formed by thefirst and second arms 204 and 208 of the C-frame 202. The anvil furtherincludes a surface 207 which is substantially perpendicular to the axis216 of the anvil 206. A ceramic disk 100a having a diamond coatedcontact surface 106a, such as shown and described above with referenceto FIG. 1, is attached to the surface 207 of the anvil 206 with thediamond coated contact surface 106a exposed. The spindle mechanismincludes an axially adjustable cylindrical spindle 222, a spindlehousing 224 in which the spindle 222 is movably engaged, a spindleadjuster 226, and a precision adjuster 226a. The spindle 222 has a firstend 228 parallel to and facing the anvil surface 207, and a second end232 rotatably engaged to the spindle adjuster 226, such that when thespindle adjuster 226 is rotated, the spindle 222 moves linearly alongthe axis 216 of the anvil 206. A ceramic disk 100b having a diamondcoated contact surface 106b, such as shown and described above withreference to FIG. 1, is attached to the first end 228 of the spindle 222so that the contact surface 106b faces the contact surface 106a. Thesecond end 232 of the spindle 222 and the spindle adjuster 226 bothcontain measuring indicia 234,236. The precision adjuster 226a is acircular grooved disk that can be rotated to achieve smaller incrementsin spindle movement than that achieved by the spindle adjuster 226.

It will be appreciated that during measurement of an object, when thespindle 222 is moved relative to the anvil 206 to grasp the objecttherebetween, the electronics package measures the movement of thespindle 222 relative to the anvil 206 and provides an indication of thedistance between the anvil and the spindle. It will also be appreciatedthat the electronics package will be calibrated to indicate the distancebetween the facing surfaces 106a, 106b of the anvil and spindle disks100a, 100b. The distance is displayed on an LCD screen 212a of thedigital readout 212. The associated keypad 214 which includes on/offkeys 214a, 214b, a mode key 214c which selects the units of measurement,and a hold key 214d which saves any recorded measurement is used inmanners known in the art. With the provided arrangement, the diamondcoated disks 100a, 100b provide a precise, substantially wear-resistantcontact surface for the digital micrometer.

Turning now to FIG. 3, a detailed view of a diamond coated contact edge300 for the measuring jaw of a calipers is shown. The contact edge 300includes a silicon nitride rectangular strip 302. The strip 302 has aside edge 304, and also includes a first surface 306 opposite andparallel a second surface 308. The strip is prepared so that itssurfaces 306 and 308 are parallel to each other within 0.25 microns. Adiamond layer 310 having a preferred thickness of about ten microns iscoated on the first surface 306 of the strip and preferably over theedge 304 by any known CVD technique. The second surface 308 of the strip302 is metalized for affixation to the first and second outsidemeasuring jaws of a calipers by brazing, gluing, welding, or any othersuitable technique. In accord with the invention, the strip 302 may bemade of ceramic compounds other than silicon nitride, such as siliconcarbide or tungsten carbide, provided the ceramic compound can beproperly coated with a diamond layer and is sufficiently strong to actas a contact edge.

Referring to FIG. 4, a second exemplary embodiment of a measuringinstrument 400 is shown. The measuring instrument 400, a precisioncalipers, includes a track 402 having first and second edges 404, 406, afixed first outside measuring jaw 408 perpendicular to the edges 404,406 of the track 402, a fixed first inside measuring jaw 410perpendicular to the edges 404, 406 of track 402, and a measuring unit412 adjustably disposed along the track 402. Attached to the measuringunit 412 is an adjustable second outside measuring jaw 414 which isparallel to and faces the first outside jaw 408 and an adjustable secondinside measuring jaw 416 parallel to and facing the first inside jaw410. The measuring unit 412 also includes a rotating adjusting wheel418, a rotating precision knob 420, an electronics package (not shown)and an electronic keypad 422 with LCD digital readout 424 which arecoupled to the electronics package. In accord with the invention, thefirst outside measuring jaw 408 and the adjustable second outsidemeasuring jaw 414 are provided with respective diamond contact edges300a, 300b such as the kind described above with reference to FIG. 3.

When the adjusting wheel 418 is rotated, the measuring unit 412 movesalong the track 402 along with the adjustable second outside measuringjaw 414 having the diamond contact edge 300b and the adjustable secondinside measuring jaw 416. The precision knob 420 moves the measuringunit 424 along the track 402 in smaller increments than the adjustingwheel 418. Several keys 422a, 422b, 422c are available on the electronickeypad 422 for selecting the units of measurement and performingmeasurement related operations. The LCD digital readout displays datarecorded by the measuring unit 412 and entered on the keypad 422. Withthe provided arrangement, the diamond contacts 300a, 300b provide aprecise, substantially wear-resistant gripping or contact surface forthe digital calipers.

Turning now to FIG. 5, a detailed view of an alternative embodiment of adiamond coated contact edge 502 for the outside measuring jaw 500 of acalipers is shown. The contact edge 502 is formed by a portion 506 ofone end of a tungsten carbide rectangular jaw liner strip 504. The jawliner strip 504 generally has an outside edge 508, an inside edge 510,and a first surface 512 opposite and parallel a second surface 514. Thesurfaces 512 and 514 are prepared such that they are parallel to eachother within 0.25 microns. Portion 506 of the strip 502 is generallystep shaped with a lower step 518, and an upper step 516 having an uppertread 522 and a riser 524. The upper step 516 is preferably smaller inwidth than the lower step 518 and provides the contact edge of theoutside measuring jaw 500 for the calipers. The preferred dimensions (ininches) of the jaw liner strip 504 are listed below with minortolerances not listed:

    ______________________________________    d1  (strip width):                      0.135  d2  (uncovered strip                                              0.760                                 length):    d3  (strip height):                      0.060  d4  (tread length):                                              0.440    d5  (lower step height):                      0.040  d6  (lower tread width):                                              0.025    d7  (riser height):                      0.020  d8  (upper tread width):                                              0.040    ______________________________________

A free standing CVD diamond layer 520 having a preferred thickness ofabout ten microns is fitted and vacuum brazed on at least the uppertread 522 and the riser 524 of the upper step 516, although it may alsocover at least a portion of an outer edge 524a of step 516 as well. Thefree standing diamond layer 520 is manufactured by diamond coating acarrier substrate with any known CVD technique and then removing thecarrier substrate. The resulting free standing diamond is then cut tothe appropriate dimensions before being affixed to the upper step 516.

In affixing the diamond layer 520 to the upper step 516 by vacuumbrazing, it is desirable that the surface of the diamond be coplanarwith the surface 512 of the remaining portion of the liner strip 504.Three different techniques can be utilized to accomplish this. In afirst technique, after the diamond is coated on the upper step 516, thediamond layer 520 on the upper step surface 522 is slightly higher thanthe liner strip surface 512, which is preferably a hard ceramic such astungsten carbide. The diamond layer 520 is then finished to make itcoplanar with the tungsten carbide surface 512 of the rest of the jawliner strip 504. In a second technique, after the diamond layer 520 iscoated on the upper step 516, the tungsten carbide surface 512 is higherthan the diamond surface. Thus, the tungsten carbide surface 512 isfinished so as to be coplanar with the diamond layer. This secondtechnique may be preferable to the first technique in light of thehardness of diamond as compared to that of tungsten carbide. A thirdalternative technique is to guarantee that the thickness of the brazingmaterial (not shown) used to affix the diamond layer 520 is such thatthe diamond layer 520 coating will be flush and coplanar with thetungsten carbide surface 512 once affixed to the upper step 516 of thefirst end 506.

Regardless of the technique utilized to guarantee that the diamond iscoplanar with the remainder of the tungsten carbide surface 512, thesecond surface 514 of the strip 504 is metalized for affixation to theoutside measuring jaw 500. The strip 504 may be affixed to the measuringjaw 500 by gluing, such as with a cyanoacrylate glue, or by brazing,welding, or any other suitable technique. In accord with the invention,the strip 504 may be made of ceramic compounds other than tungstencarbide, such as silicon carbide or silicon nitride, provided theceramic compound can be properly coated with a diamond layer and issufficiently strong to act as a contact edge.

According to a first method of the invention, a diamond coated contactis made by first shaping a silicon nitride or other suitable substrateso that it has a first surface and an opposite second surface which areparallel to each other within 0.25 microns. The substrate is preferablysized and shaped (e.g., circular, rectangular, etc.) to fit the contactsurface of a measuring instrument. A diamond layer is then coated on thefirst surface of the substrate to a preferred thickness of about tenmicrons by using any one of several known chemical vapor depositionprocesses (CVD). The second surface is then metalized before beingaffixed by gluing, brazing, welding, or any other suitable technique tothe measuring instrument. The same diamond coating and metalizingprocess is repeated with another ceramic substrate to provide a secondcontact for the measuring instrument. If desired, one or more edgesurfaces of the ceramic substrate may also be coated with diamond via aCVD process.

According to the second method of the invention, a diamond layer issimilarly CVD coated but onto a removable substrate. Once coated, thesubstrate is removed, leaving a free standing diamond layer. This freestanding diamond layer is vacuum brazed onto a first surface of aceramic strip. The second surface of the ceramic strip is thenmetallized and affixed to the measuring instrument by any techniquedescribed in the first method of the invention.

There have been described and illustrated herein several embodiments ofa measuring instrument having diamond coated contacts and a method ofdiamond coating the contacts of these measuring instruments. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. Thus, while particular measuringinstruments with the diamond coated contact surfaces such as micrometersand calipers have been disclosed, it will be appreciated that othermeasuring instruments could have diamond contacts as well. Furthermorewhile particular types of ceramic substrates such as silicon nitride,silicon carbide and (cemented) tungsten carbide have been disclosed, itwill be understood that any suitable substrate can be used. For example,and not by way of limitation, other ceramics such as zirconia alumina,and alumina, and hard metals such as tungsten can be used provided theycan be formed to a parallel surface and can be diamond coated by adiamond layering process. Also, while a ten micron thick diamond layercoating on the entire exposed ceramic substrate is preferred, it will berecognized that the diamond layer may be of other desired thicknesses(e.g., one micron or more) and that the substrate may be only partiallycoated. Moreover, while particular configurations have been disclosed inreference to the dimensions and shape of the ceramic substrate, it willbe appreciated that other configurations could be used as well.Furthermore, while a CVD process has been disclosed for coating thesurface contacts, it will be understood that any diamond layermanufacturing method can be similarly used. It will therefore beappreciated by those skilled in the art that yet other modificationscould be made to the provided invention without deviating from itsspirit and scope as so claimed.

We claim:
 1. A measuring instrument for measuring an object,comprising:first and second opposed contact members with respectivefirst and second contact faces, at least said first contact memberhaving an end portion comprising a hard substrate fixed to said contactmember and coated with a diamond film coating on at least a portion ofan exposed surface of said hard substrate to form said first contactface.
 2. A measuring instrument according to claim 1, wherein:said hardsubstrate is a first ceramic substrate, wherein said first ceramicsubstrate is coated with said diamond film coating.
 3. A measuringinstrument according to claim 2, wherein:said first ceramic substrate isa material selected from the group consisting of silicon nitride,silicon carbide, tungsten carbide, zirconia alumina, and alumina.
 4. Ameasuring instrument according to claim 2, wherein:said measuring deviceis a micrometer having a fixed anvil having said first opposed contactmember, a spindle having said second opposed contact member, said secondopposed contact member being adjustable relative to said first opposedcontact member.
 5. A measuring instrument according to claim 4,wherein:said ceramic substrate is of a substantially constant thicknesswhich varies at most by 0.25 microns.
 6. A measuring instrumentaccording to claim 5, wherein:said diamond film coating is at leastsubstantially ten microns thick.
 7. A measuring instrument according toclaim 2, wherein:said second contact member has an end portioncomprising a second hard substrate fixed to said second contact memberand coated with a diamond film coating on at least a portion of anexposed surface of said second hard substrate to form said secondcontact face.
 8. A measuring instrument according to claim 7,wherein:said measuring device is a micrometer having a fixed anvilhaving said first contact member, a spindle having said second contactmember, said second contact member being adjustable relative to saidfirst contact member.
 9. A measuring instrument according to claim 8,wherein:said first ceramic substrate is of a substantially constantthickness which varies at most by 0.25 microns, and said second ceramicsubstrate is of a substantially constant thickness which varies at mostby 0.25 microns, and said first diamond film coating is at leastsubstantially ten microns thick, and said second diamond film coating isat least substantially ten microns thick.
 10. A measuring instrumentaccording to 2, wherein:said measuring device is a calipers having afirst outside measuring jaw having said first contact member and asecond outside measuring jaw having said second contact member, saidsecond contact member being adjustable relative to said first contactmember.
 11. A measuring instrument according to claim 10, wherein:saidfirst ceramic substrate is of a substantially constant thickness whichvaries at most by 0.25 microns.
 12. A measuring instrument according toclaim 11, wherein:said diamond film coating is at least substantiallyten microns thick.
 13. A measuring instrument according to claim 7,wherein:said measuring device is a calipers having a first outsidemeasuring jaw having said first contact member and a second outsidemeasuring jaw having said second contact member, said second contactmember being adjustable relative to said first contact member.
 14. Ameasuring instrument according to claim 13, wherein:said first ceramicsubstrate is of a substantially constant thickness which varies at mostby 0.25 microns, and said second ceramic substrate is of a substantiallyconstant thickness which varies at most by 0.25 microns, and said firstdiamond film coating is at least substantially ten microns thick, andsaid second diamond film coating is at least substantially ten micronsthick.
 15. A measuring instrument for measuring an object,comprising:first and second opposed contact members with respectivefirst and second contact faces, at least said first contact memberhaving a hard substrate fixed to said contact member, said hardsubstrate having a main portion and an end portion with a diamond filmcoating on at least a portion of said end portion, said diamond filmcoating being substantially coplanar with said main portion of said hardsubstrate, said diamond film coating forming at least a portion of saidfirst contact face.
 16. A measuring instrument according to claim 15,wherein:said end portion of said hard substrate is stepped, with a topstep having a tread and a riser, said tread and said riser having saiddiamond film coating thereon, wherein said tread is substantiallycoplanar with said main portion of said hard substrate.
 17. A measuringinstrument according to claim 16, wherein:said main portion of said hardsubstrate has a first width, and said tread of said top step has asecond width smaller than said first width.
 18. A measuring instrumentaccording to claim 15, wherein:said diamond film coating is at leastsubstantially ten microns thick.
 19. A measuring instrument according toclaim 16, wherein:said diamond film coating is a free standing diamondlayer which is brazed to at least said tread, and said hard substrate ismetallized and affixed to said first contact member.